Method and installation for separating a mixture of hydrogen and carbon monoxide

ABSTRACT

The invention relates to a method for simultaneously producing hydrogen and carbon monoxide, whereby the synthesis gas is subjected to decarbonatation in a decarbonatation unit ( 2 ), and to desiccation in a desiccation unit ( 5 ). The remaining constituents are cryogenically separated and a hydrogen-enriched gas ( 7, 19, 21 ) is recycled upstream from the decarbonatation unit and downstream from a unit for producing synthesis gas (F).

The present invention relates to a method and an installation forseparating a mixture of hydrogen and carbon monoxide. In particular, itrelates to a method for separating such a mixture using a step ofseparation by cryogenic distillation.

Carbon monoxide and hydrogen production units can be divided into twoparts:

-   -   Generation of synthesis gas (mixture primarily containing H₂,        CO, CH₄, CO₂ and N₂). Among the various industrial methods for        producing synthesis gas, steam reforming is the most important.        The design of this unit, which comprises a furnace, is based on        the required production of CO and hydrogen.    -   Production of synthesis gas. This comprises:    -   an amine scrubbing unit to remove most of the CO₂ present in the        synthesis gas;    -   a unit for purification on adsorbent bed. This unit generally        comprises two bottles in continuous operation, one in        production, the second in regeneration phase;    -   a unit for low temperature treatment by a cryogenic method (cold        box) in order to produce carbon monoxide and hydrogen (possibly        including a mixture of carbon monoxide and hydrogen called        Oxogaz) in the quantities and purities required by the consumer.        The most common method is scrubbing with liquid methane to        obtain pure carbon monoxide with a recovery yield up to 99%,        hydrogen the CO content of which generally varies between a few        ppm and 1%, and a methane rich waste gas used as fuel.

Methods of this type are described in “Tieftemperaturtechnik” by Hausenet al., Springer-Verlag 1985 pp 417-419, EP-A-837031, EP-A-0359629,EP-A-0790212 and EP-A-1245533.

The thermodynamic equilibrium of the synthesis gas generation unit isfavored by low pressure, which results in a lower consumption of rawmaterial, while the synthesis gas purification unit is favored by highpressure in terms of equipment size and electric power consumption.

This is why, and due to the limitation of the operating pressure ofreforming furnaces (which operate at a pressure below 45 bar abs.), itmay be advantageous and/or necessary to incorporate a synthesis gascompressor in the synthesis gas purification line.

In most cases, the hydrogen produced by the cold box, containing up to 1mol % of CO, is used as regeneration gas for the purification, and isthen sent to an adsorbent purification unit (PSA) before being sent tothe end consumer.

In the case in which the hydrogen produced by the cold box is sentdirectly to the consumer with a CO content specification of a few ppm,this gas can no longer be used as regeneration gas.

Also in the case in which a mixture of carbon monoxide and hydrogen isproduced, generally containing 50% hydrogen, the quantity of hydrogenremaining as waste gas is too small to regenerate the purification; itis therefore necessary to find another gas as a regeneration gas.

One of the current solutions is to produce a necessary quantity ofadditional hydrogen in the generation unit. This hydrogen present in thesynthesis gas is treated in the purification unit and particularly inthe methane scrubbing unit, is then used as regeneration gas for thepurification, and finally utilized as fuel.

One subject of the invention is a method for simultaneously producinghydrogen and carbon monoxide, of the type in which a synthesis gas isreceived, such as a gas from hydrocarbon reforming, containing hydrogenand carbon monoxide, from a synthesis gas production unit, the synthesisgas is decarbonated in a decarbonation unit, and desiccated in adesiccation unit, followed by cryogenic separation of the remainingcomponents, characterized in that a gas containing at least 60% hydrogenconsisting of:

-   -   (i) a gas from the cryogenic separation and/or    -   (ii) a part of the synthesis gas is recycled upstream of the        decarbonation unit and downstream of the synthesis gas        production unit.

According to other optional aspects of the invention,

-   -   the gas containing at least 60% hydrogen is withdrawn at the top        of a methane scrubbing column of the cryogenic separation unit,        in which the remaining components are separated;    -   the gas containing at least 60% hydrogen is a portion of the gas        with the highest hydrogen purity produced;    -   the gas containing at least 60% of hydrogen is used to        regenerate the desiccation unit before being sent upstream of        the decarbonation unit;    -   the synthesis gas purified in the decarbonation unit is        compressed in a compressor before being sent to the desiccation        unit;    -   another gas enriched with hydrogen is sent from the cryogenic        separation upstream of the compressor and downstream of the        decarbonation unit.

A further aspect of the invention provides for an installation forsimultaneously producing hydrogen and carbon monoxide comprising asynthesis gas production unit, a decarbonation unit, a desiccation unitand a cryogenic separation unit, and means connecting the synthesis gasproduction unit with the decarbonation unit, the decarbonation unit withthe desiccation unit and the desiccation unit with the cryogenicseparation unit, and means for withdrawing hydrogen and carbon monoxideas products, characterized in that it comprises means for recycling agas containing at least 60% hydrogen consisting of:

-   -   (i) a gas enriched with hydrogen, from the cryogenic separation        unit and/or    -   (ii) a portion of the synthesis gas upstream of the        decarbonation unit and downstream of the synthesis gas        production unit.

The means for recycling the gas are preferably connected both to a pointupstream of the desiccation unit and downstream of the synthesis gasproduction unit, and at the cryogenic separation unit or a pointupstream of the cryogenic separation unit.

According to other optional aspects of the invention, the installationcomprises:

-   -   compression means downstream of the decarbonation means.    -   means for sending the hydrogen enriched gas to the desiccation        unit.    -   means for sending a gas enriched with hydrogen from the        stripping column downstream of the decarbonation unit.

The cryogenic separation unit can comprise a methane scrubbing column, astripping column, a rectifying column and means for withdrawing thehydrogen enriched gas from the methane scrubbing column. Other types ofunit can be considered, such as a partial condensation unit.

In the case of the presence of a synthesis gas compressor, theinnovation proposed consists in installing a hydrogen rich gas recycleloop between the cold box and upstream of the amine scrubbing unit.

This hydrogen rich gas, produced at the outlet of the cold box by theliquid methane scrubbing column, is used as purification regenerationgas, expanded and sent upstream of the amine scrubbing unit, to be mixedwith the synthesis gas from the generation unit.

No excess hydrogen must be produced.

This has the result of reducing the size of the synthesis gas generationunit by about 5% to 15%.

Another advantage is the recovery of the quantity of CO co-adsorbed inthe purification unit, which returns to the synthesis gas loop. This hasthe result of increasing the carbon monoxide recovery rate by about0.5%.

The flash gas from the cold box can also be recycled upstream of thesynthesis gas compressor to improve the CO yield of the unit.

All the percentages given herein are molar percentages and the pressuresare absolute pressures.

The invention will now be described in greater detail with reference tothe drawings, of which FIG. 1 schematically represents the separation ofthe synthesis gas by several steps including cryogenic separation, andFIG. 2 shows a cryogenic separation apparatus suitable for beingincorporated in FIG. 1.

In FIG. 1, a synthesis gas stream 1 at about 16 bar from a steamreforming furnace F is separated in an amine scrubbing unit 2 to removethe carbon dioxide. This product is then compressed in a compressor 3 toa pressure between 18 and 43 bar abs. The compressed stream 4 isstripped of water in a purification unit 5 to produce a gas flow rate of55500 Sm³/h containing 62% hydrogen, less than 1% nitrogen, 35% carbonmonoxide and 3% methane.

This stream is then separated in a cryogenic separation apparatus toproduce a gaseous product 8 of 25400 Sm³/h constituting a mixture ofcarbon monoxide and hydrogen (typically 50% hydrogen and slightly over49% carbon monoxide), a gaseous product 9 of 18700 Sm³/h rich inhydrogen (typically 99% hydrogen), a gaseous product 11 of 6500 Sm³/hrich in carbon monoxide (typically 99% carbon monoxide), a methane purge13, a hydrogen rich gas 7 and a flash gas 15 of 1300 Sm³/h (typicallycontaining 95% hydrogen, 1% carbon monoxide and 4% methane). A stream of1700 Sm³/h of stage gas 14 containing over 98% hydrogen is sent to anexpansion turbine.

The stream 7 of 6800 Sm³/h is sent to the purification unit 5 and isused to regenerate one of the adsorbent beds thereof and then, saturatedwith water, it is mixed with the synthesis gas upstream of the aminescrubbing unit 2.

Optionally, a portion 17 of the waste gas 15 can be recycled upstream ofthe compressor 3 and upstream or downstream of the amine scrubbing unit2.

The pure hydrogen product 9 is sold directly as pure product withoutpurification by a PSA unit. The streams of flash gas 15 and methanepurge 13 are too small to regenerate the purification 5. The methanepurge 13 can advantageously be sent to the inlet of the furnace F.

This recycling of hydrogen rich gas 7 serves to reduce the size of thesteam reforming furnace by nearly 10% and to increase the CO yield by0.5%.

As a variant or in addition, a portion 19 of the synthesis gas GS can beseparated downstream of the desiccation unit 5 and sent upstream of thedecarbonation unit 2. This stream 19 can also be used to regenerate thedesiccation unit 5 before being mixed with the untreated synthesis gas1.

This has the advantage of serving to reduce the size of the cold box ofthe cryogenic separation unit 6.

FIG. 2 shows an apparatus 6 for separating the synthesis gas bycryogenic distillation. The streams having the same reference numeralsas those in FIG. 1 correspond to the streams designated in FIG. 1. Theapparatus comprises a methane scrubbing column K1, a stripping column K2and a rectifying column K3. The cooled and purified synthesis gas GS issent to the bottom of the methane scrubbing column K1. Two hydrogenenriched streams are withdrawn from the column, including a stream 9 anda stream 7 withdrawn a few theoretical trays below the stream 9.

The liquid stream 20 enriched with methane and carbon monoxide isseparated into a liquid stream 22 and a two-phase stream 23 and sent tothe column K2. The stream 23 is sent directly to the column K2, whilethe stream 22 is partially vaporized (not shown) before being sent tothe column K2.

A hydrogen enriched gas 15 is withdrawn at the top of the strippingcolumn K2. At the bottom of the stripping column K2, a stream 24containing mainly carbon monoxide and methane is withdrawn, sub-cooled(not shown) and separated into two streams 25 and 26. The stream 25 issent directly to the column K3, the stream 26 is vaporized (not shown)and sent to the column K3. The carbon monoxide rich product 11 iswithdrawn at the top of the column K3. A liquid methane stream 27 iswithdrawn at the bottom of the column K3 and then pressurized in a pumpP, divided into two and sent partly to the top of the stripping columnK2 and the remainder is sent to the top of the methane scrubbing columnK1, the stream 13 constituting the methane purge.

The reboiling at the bottom of the columns K2 and K3 and thecondensation at the top of the column K3 is provided in a known mannerby a carbon monoxide cycle (not shown).

1-11. (canceled)
 12. A method which may be used for simultaneouslyproducing hydrogen and carbon monoxide, said method comprising: a)receiving a synthesis gas from a synthesis gas production unit, whereinsaid synthesis gas comprises hydrogen and carbon monoxide; b)decarbonating said synthesis gas in a decarbonation unit; c) desiccatingsaid synthesis gas in a desiccation unit; d) cryogenically separatingsaid desiccated synthesis gas in a cryogenic separation unit; and e)recycling a gas, containing at least about 60% hydrogen, upstream ofsaid decarbonation unit and downstream of said synthesis gas productionunit, wherein said gas to be recycled comprises at least one memberselected from the group consisting of: 1) a gas from said cryogenicseparation; and 2) a portion of a gas upstream of said cryogenicseparation unit.
 13. The method of claim 12, wherein said portion of agas upstream of said cryogenic separation unit comprises a portion ofsaid desiccated synthesis gas.
 14. The method of claim 12, wherein saidsynthesis gas comprises a gas from hydrogen reforming.
 15. The method ofclaim 12, further comprising withdrawing said gas to be recycled fromthe top of a methane scrubbing column in said cryogenic separation unit.16. The method of claim 12, wherein the gas produced with the highesthydrogen purity comprises said gas containing at least about 60%hydrogen.
 17. The method of claim 12, further comprising regeneratingsaid desiccation unit with said gas containing at least about 60%hydrogen prior to sending said gas containing at least about 60%hydrogen upstream of said decarbonation unit.
 18. The method of claim ofclaim 12, further comprising compressing said decarbonated synthesis gasin a compressor prior to sending said decarbonated synthesis gas to saiddesiccation unit.
 19. The method of claim 17, further comprising sendinga second gas to a location upstream of said compressor and downstream ofsaid decarbonation unit, wherein said second gas is enriched withhydrogen from said cryogenic separation.
 20. An apparatus which may beused for substantially simultaneously producing hydrogen and carbonmonoxide, said apparatus comprising a) a means for receiving a synthesisgas from a synthesis gas production unit; b) a decarbonation unit; c) adesiccation unit; d) a cryogenic separation unit; e) a means forconnecting said synthesis gas production unit with said decarbonationunit; f) a means for connecting said decarbonation unit with saiddesiccation unit; g) a means for connecting said desiccation unit withsaid cryogenic separation unit; and h) a means for withdrawing hydrogenand carbon monoxide as products, wherein said withdrawing meanscomprises a means for recycling a gas containing at least about 60%hydrogen, wherein said gas to be recycled comprises at least one memberselected from the group consisting of: 1) a gas from said cryogenicseparation; and 2) a portion of a gas upstream of said cryogenicseparation unit.
 21. The apparatus of claim 20, wherein said portion ofa gas upstream of said cryogenic separation unit comprises a portion ofsaid desiccated synthesis gas.
 22. The apparatus of claim 20, whereinsaid products are withdrawn upstream of said decarbonation unit and downstream of said synthesis gas production unit.
 23. The apparatus of claim20, further comprising a compression means located downstream of saiddecarbonation means.
 24. The apparatus of claim 20, further comprising ameans for sending said hydrogen enriched gas to said desiccation unit.25. The apparatus of claim 20, wherein said cryogenic separation unitcomprises: a) a methane scrubbing column; b) a stripping column; c) arectifying column; and d) a means for withdrawing said hydrogen enrichedgas from said methane scrubbing column.
 26. The apparatus of claim 25,further comprising a means for sending a hydrogen enriched gas from thestripping column to a location downstream of said decarbonation unit.